Color LCDs most commonly in contemporary general use comprise a two-dimensional array of display elements, each element including red (R), green (G) and blue (B) sub-pixels employing associated color filters. Each such element is operable to display potentially all colors, but the color filters of each element absorb in the order of ⅔ of light passing through it. In order to increase element optical transmittance, it is known practice in the art to add a white sub-pixel (W) to each element in a manner as depicted in FIG. 1 wherein a three-sub-pixel element is indicated by 10, and a four-sub-pixel element including a white (W) sub-pixel is indicated by 20.
In the element 20, the red (R), green (G) and blue (B) sub-pixels each have an area which is 75% of that of a corresponding color sub-pixel included in the element 10. However, the white (W) sub-pixel of the element 20 does not include a color filter therein and in operation is able to transmit an amount of light corresponding to a sum of light transmissions through the red (R), green (G) and blue (B) sub-pixels of the element 20. Thus, the element 20 is capable of transmitting substantially 1.5 times more light than the element 10. Such enhanced transmission is of benefit in LCDs employed to implement television, in lap-top computers where increased display brightness is desired, in projection television (rear and front view, LCD and DLP), in lap-top computers where increased display brightness is desired, in lap-top computers where highly energy-efficient back-lit displays are desired to conserve power and thereby prolong operating time per battery charge session, and in LCD/DLP graphics projectors (beamers). However, introduction of the white (W) sub-pixel into the element 10 to generate the element 20 introduces a technical problem regarding optimal drive to the R, G, B, W sub-pixels of each element 20 to provide optimal rendition of a color image on the display.
Liquid crystal displays (LCDs) each comprising an array of elements, wherein each element includes red (R), green (G), blue (B) and white (W) sub-pixels, are described in a published U.S. patent application No. US2004/0046725. Moreover, the displays described each also includes gate lines for transmitting gate signals to their sub-pixels, and data lines for transmitting data signals to their sub-pixels. The displays described each further includes a gate driver for supplying gate signals to the gate lines, a data driver for supplying data voltages to the data lines, and an image signal modifier. The image signal modifier includes a data converter for converting three-color image signals into four-color image signals, a data optimizer for optimizing the four-color image signals from the data converter, and a data output unit supplying the optimized image signals to the data driver in synchronization with a clock.
Regimes for driving the four red (R), green (G), blue (B), and white (W) sub-pixels of each element are known. In a known “Min-simple” regime, such regime representing a simplest driving method, display input signals Ri, Gi, Bi for red, green, blue colors respectively are mapped to corresponding output signals for driving red (R), green (G), blue (B) sub-pixels respectively, these output signals being denoted by Ro, Go, Bo respectively. In the “Min-simple” regime, a minimum of the input signals Ri, Gi, Bi is computed for each element to generate a drive signal Wo for the white (W) sub-pixel thereof. In this “Min-simple” regime, a first set of equations (Eqs. 1) pertain:Wo=min(Ri, Gi, Bi) Ro=RiGo=Gi Bo=Bip  Eqs. 1wherein min(x, y, z) is a function identifying a minimum value of arguments x, y and z. When the first set of equations (Eqs. 1) is employed, the input signals Ri, Gi, Bi=240, 160, 120 respectively results in the output signals such that Ro, Go, Bo, Wo=240, 160, 120, 120 respectively. A total RGB optical color output from all four sub-pixels of the element 20 then becomes Rt, Gt, Bt=360, 280, 240. A comparison of the input signals Ri, Gi, Bi to the optical color achieved Rt, Gt, Bt shows an enhanced brightness but with a decreased color saturation for all but white, grey and fully saturated colors in an image presented; such distortion of color rendition represents a technical problem addressed by the present invention.
In another known regime denoted by “Min−1”, the output signals Ro, Go, Bo are modified in order to keep the ratio between R, G, B constant. A maximum value for the output signals Ro, Go, Bo is not changed by such an approach, but values of non-maximal components do become modified. In the “Min−1” regime, a set of equations (Eqs. 2) pertains:Max=max(Ri, Gi, Bi) Min=min(Ri, Gi, Bi) Wo=MinRo=[Ri*(Wo+Max)/Max]−Wo Go=[Gi*(Wo+Max)/Max]−Wo Bo=[Bi*(Wo+Max)/Max]−Wo  Eqs. 2
For example, the input signals Ri, Gi, Bi=240, 160, 120 respectively result in the output signals Ro, Go, Bo, Wo=240, 120, 60, 120 respectively resulting in a total color output of Rt, Gt, Bt=360, 240, 180 respectively. This “Min−1” regime provides enhanced brightness whilst maintaining correctly a ratio between colors, thus color saturation does not change. Hence, the “Min−1” regime is operable to provide more satisfactory results in comparison to the aforementioned “Min−simple” regime.
In the “Min−1” regime, a value for the output Wo for the white (W) sub-pixel is simply derived from a minimum of the input signals Ri, Gi, Bi. Known “Min−2” and “Min−3” regimes are similar to the “Min−1” regime except that the output Wo for the white (W) sub-pixel is calculated from Equation 3 (Eq. 3) and Equation 4 (Eq. 4) respectively:Wo=255 (Min/255)2  Eq. 3Wo=−Min3/255+Min2/255+Min  Eq. 4
The “Min−2” regime is operable to enhance highlights in color images presented on a corresponding LCD, whereas the “Min−3” regime is operable to enhance mid-tones in images presented on the LCD.
Alternatively, in a “MaxW” regime derived from the aforementioned “Min−1” regime, a value for the output Wo for driving the white (W) sub-pixel is derived from conditions as defined in Equations 5 (Eqs. 5):Wo=(Min*Max)/(Max−Min) when min/max<=0.5Wo=Max when min/max>0.5  Eqs. 5
For example, when using the MaxW regime, the input signals having values Ri, Gi, Bi=240, 160, 120 respectively result in the outputs Ro, Go, Bo, Wo=240, 80, 0, 240 respectively and consequently total observed color ratios Rt, Gt, Bt=480, 320, 240 respectively; in other words, brightness is enhanced and color saturation is maintained.
In a published article “TFT-LCD with RGBW Color System”, Baek-woon Lee et al., Samsung Electronics Corp., Society for Information Display 2003—Digest of Technical papers, pp. 1212-1215, there is described an alternative regime to the aforesaid MaxW regime; in the alternative regime disclosed, an output for the white (W) sub-pixel is not defined and the total color output Rt, Gt, Bt is determined directly from the input signals Ri, Gi, Bi respectively pursuant to Equations 6 (Eqs.6):Gain=1+Min/(Max−Min) such that Gain is limited to a value 2Rt=Ro+Wo=Gain*Ri Gt=Go+Wo=Gain*Gi Bt=Bo+Wo=Gain*Bi  Eqs. 6
For the total colors presented by the element 20, the Rt, Gt, Bt color values are identical to that which is achievable from the aforementioned MaxW algorithm, although a specific partitioning of drive between the outputs Ro, Go, Bo and Wo is not explicitly accommodated. The formulae in Equation 6 (Eqs. 6) assume equal areas of the R, G, B, W sub-pixels in the element 20. If a parameter w is a ratio of the area of the white (W) sub-pixel in the element 20 to that of the red (R), green (G), blue (B) sub-pixels thereof, then Equations 6 (Eqs. 6) taking the parameter w into account become Equations 7 (Eqs. 7) as follows:Gain=1+Min/(Max−Min) such that Gain is limited to a value 1+wRt=Ro+w*Wo=Gain*Ri Gt=Go+w*Wo=Gain*Gi Bt=Bo+w*Wo=Gain*Bi  Eqs. 7
In the regime employed by Samsung, it will be appreciated, for example, that for a red (R) region of a presented image represented in the input signal by Ri, Gi, Bi equal to 255, 0, 0 respectively, the regime cannot provide display enhancement. However, a less intense red region represented by the input signal, for example Ri, Gi, Bi represented by 128, 0, 0 respectively, is potentially susceptible to enhancement although it is not enhanced in such case.
The inventors have appreciated that although inclusion of the white (W) sub-pixel in the element 20 is capable of increasing corresponding display brightness, various known regimes for driving the four sub-pixels of the element 20 to obtain an optimal compromise between enhanced brightness and best color rendition suffer technical problems of overall image color rendition. The inventors have therefore devised alternative approaches for driving sub-pixels of the element 20 to at least partially address these technical problems.